This invention relates to a method for performing a keratotomy and an apparatus for the same.
Keratotomy is the surgical reshaping of the cornea of the eye with a series of radial and/or arcuate cuts that are made in the cornea. The incisions flatten the cornea. This decreases its radius of curvature. Radial cuts correct myopia while arcuate cuts correct astigmatism. It is common to correct both myopia and astigmatism during the same keratotomy procedure.
In FIG. 1 an eye 10 is illustrated schematically. Its outer shape is defined by the cornea 14 and the sclera 16. The cornea 14 is a transparent curved member that is a major site of refraction of light entering the eye. On its perimeter the cornea 14 is connected to the sclera. As is well known, within the eye and behind the cornea are the lens 18 and the iris 22.
The cornea is thinnest at its central area and thicker where it joins the sclera with the thickness ranging from about 0.5 mm in the central area to about 0.8 mm on the periphery. The cornea typically has a diameter of about 11.0 mm to 12.0 mm.
The eye 10 includes a visual axis 24 which passes through the centers of curvature of the inner and outer surfaces of the cornea 14. It also includes a central area having a diameter of about 3.0 mm that is called the optical zone 26.
As presently practiced, the radial cuts are made along lines that pass through the visual axis 24. However, the inner limit of the cut is defined by the optical zone 26. There may be between four and eight radial cuts. The number of cuts and their length are determined by several factors including the age of the patient, the size of the cornea, its thickness and the extent to which the vision is to be corrected. The cuts should not be in the optical zone because they will cause scaring and glare.
The arcuate cuts are made at distance that is about 3.5 mm radially outwardly from the visual axis 26. They are located between the radial cuts, but do not intersect them. Usually there are two arcuate cuts that are diametrically opposed across the visual axis.
The cuts are usually made with a knife having a blade whose depth of cut can be adjusted with micrometer precision. The blades are set so that both the radial cuts and arcuate cuts penetrate about ninety percent of the corneal thickness.
The thickness of the cornea at the outer edge of the optical zone 26 is measured. Usually a pachymeter, which employs ultrasound, is used. Then, the ophthalmic surgeon, using a knife having the thinnest possible blade, sets the depth of the knife blade and makes the radial and/or arcuate cuts as is appropriate. Usually, only one measurement of corneal thickness is made. Nonetheless, it maybe desirable to measure the thickness at several locations since the cornea may have areas of different thickness.
As currently practiced, the keratotomy procedure relies upon marks made on the cornea by the ophthalmic surgeon that indicate where the radial keratotomy cuts are to be made. There may be a risk of inaccuracy and/or harm to the patient. This is because the surgeon is cutting freehand along the marks. Therefore, the actual cuts may deviate from the marks on the cornea. Also, there is risk that the cuts may enter the optical zone and/or they will extend too far toward the sclera, or be too deep and perforate the cornea. The problem is aggravated by the thinness of the blade since its thinness makes it more difficult to control.
Further, when arcuate cuts are made so as to correct astigmatism, there is a risk that the arcuate cuts will intersect the radial cuts since their direction is transverse to the radial cuts thereby distorting vision rather than improving it.
It would be desirable to provide a method and apparatus which simplifies the keratotomy procedure so that the time that it takes can be reduced while its accuracy and safety can be increased.
Thus, with the foregoing in mind the invention relates to an apparatus for performing a keratotomy that comprises a plurality of interchangeable plates. One of the plates is retained on the eye during the keratotomy. The interchangeable other plates support corneal thickness sensors and guides for a keratotomy knife that enable both radial and arcuate cuts to be made in predetermined locations and at predetermined lengths and depths.
In another aspect the invention relates to a plate for use in a keratotomy apparatus that includes inner and outer annular members and a plurality of radially directed keratotomy knife guides that are supported by the annular members.
In still a further aspect of the invention there is provided a keratotomy plate that comprises a plurality of arcuate guides for a keratotomy knife and means for supporting the arcuate guides so that they are located outside of the optical zone when the plate is supported on the eye.
Yet a further aspect of the invention relates to the method of performing a keratotomy which comprises the steps of locating the optical axis of the eye, providing a plurality of keratotomy knife guides, placing the keratotomy knife guides on the cornea with the keratotomy knife guides being spaced from the optical zone of the eye, and making cuts in the cornea at the locations defined by the keratotomy knife guides.